The present invention relates to an image detection apparatus for use in observing or measuring a minute structure or a three-dimensional structure of a sample.
A Nipkow type confocal microscope, which uses a so-called Nipkow disk having many pinholes arranged in a spiral with an equal pitch, is well known as a confocal microscope which is a type of an image detection apparatus.
The confocal microscope obtains a confocal image of a sample by optically scanning the sample, while rotating the Nipkow disk by means of a motor. Accordingly, if a confocal image of the sample is picked up by a CCD camera, etc., a brightness/darkness fringe may occur in a picked-up image due to non-uniform scanning unless the scan cycle of the disk is synchronized with the image pick-up cycle of the CCD camera.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 9-297267, for example, discloses a motor control apparatus for a confocal microscope. FIG. 1 shows a schematic structure of the motor control apparatus. A vertical sync signal extraction circuit 2 extracts a vertical sync signal from an NTSC (National Television System Committee) type signal output from a CCD camera 1. A frequency multiplier circuit 3 multiplies the vertical sync signal and produces a control signal for a motor drive circuit 4. The motor drive circuit 4 drives a motor 5 at a speed corresponding to the control signal. Thereby, the disk speed for scanning the surface of the sample is synchronized with the image pick-up cycle of the CCD camera 1.
Since this motor control apparatus can control the rotation of the motor 5 with use of the NTSC signal from the CCD camera 1, even if the NTSC signal fluctuates, the rotation of the motor 5 can be synchronized in accordance with NTSC signal.
Jpn. Pat. Appln. KOKAI Publication No. 9-80315 discloses another apparatus for solving the problem due to non-uniform scanning. In this apparatus, scan start detection pinholes are provided at beginning points of scan tracks on the disk. A photodetector photo-electrically converts light, which has passed through the scan start detection pinhole, and synchronizing means produces a trigger signal for an image pick-up apparatus. Thereby, the scan cycle is synchronized with the image pick-up cycle to solve the problem of non-uniform scanning.
FIG. 2 shows a schematic structure of this apparatus. A scan disk (Nipkow disk in this example) 7, as shown in FIG. 3, has scan tracks 13 (indicated by hatching) provided with many pinholes like a conventional Nipkow disk, and scan start detection pinholes 14 arranged on a circumferential area of the scan tracks 13 at scan beginning points. A photodiode 8 serving as a photodetector is disposed at such a position as to receive light passing through the scan start detection pinholes 14. A current/voltage converter circuit 9 converts a current produced by the photodiode 8 to a voltage. A voltage comparison circuit 10 compares a signal from the current/voltage converter circuit 9 with a reference voltage 11 and digitizes the magnitude of the signal voltage. The resultant digital signal is output to an externally connected image pick-up apparatus 12 as an imaging sync signal (a trigger signal).
In the above structure, incident light 6 is radiated on the surface of the Nipkow disk 7. In this case, a diameter r of a beam of incident light 6 is set such that the light 6 can illuminate both the scan tracks 13 and scan start detection pinhole 14. In this state, the Nipkow disk 7 is rotated, and the incident light which has passed through the scan track 13 scans the sample in a multi-point scanning manner. On the other hand, the incident light which has passed through the scan start point detection pinhole 14 is received by the photodiode 8 each time the scan start detection pinhole 14 passes over the photodiode 8.
In this case, an output current from the photodiode 8 varies in a pulsating manner. The variation in current is converted to a voltage in the current/voltage converter circuit 9. A voltage signal from the converter circuit 9 is input to the voltage comparison circuit 10 and compared with a predetermined reference voltage 11. Thus, a signal of a voltage pulse sequence having the cycle synchronous with the scan cycle is output. This pulse-sequence signal is output to the external image pick-up apparatus 12 as a trigger signal. Thereby, the rotational cycle of the Nipkow disk 7 can be synchronized with the imaging cycle of the image pick-up apparatus 12.
In the structure disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-297267, the vertical sync signal is extracted from the NTSC signal output from the CCD camera 1 which images the surface of the sample, the extracted vertical sync signal is multiple, and the multiplied signal is output to the motor drive circuit 4 as the motor control signal. Since the signal flows in one direction in the respective components, a fluctuation in the rotation of the disk, which may occur due to eccentricity of the disk or friction of the motor shaft, cannot be fed back. Even if the frequency multiplier circuit 3 and motor drive circuit 4 perform the control based on the NTSC signal from the CCD camera 1, once a fluctuation occurs in the rotation of the disk due to the eccentricity of the disk or friction of the motor shaft, the scan cycle of the disk becomes asynchronous with the imaging cycle of the CCD camera 1 and a brightness/darkness fringe may occur in the screen image.
Besides, in the structure disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-80315, the scan start detection pinholes 14 are formed at the beginning points of the scan tracks on the Nipkow disk 7. The light which has passed through the scan start detection pinhole 14 is detected to produce the trigger signal for the image pick-up apparatus 12. On the basis of the trigger signal, the imaging timing of the image pick-up apparatus 12 is controlled and the vertical sync signal of the image signal is reset. In this case, however, if a fluctuation occurs in the rotation of the disk due to eccentricity of the disk or friction of the motor shaft, the scan start detection pinhole 14 cannot pass, at regular cycles, over the photoelectric converter 8 for generating the trigger signal. Because of this, the cycle of the trigger signals applied to the image pick-up apparatus 12 becomes irregular and also the cycle of the vertical sync signals of image signals output from the image pick-up apparatus 12 varies. Consequently, images on the TV monitor are flickering and the observation of the sample cannot be performed.